Inexpensive and low-maintenance electric current measurement devices suitable for placement in many circuit locations are useful in “smart-grid” monitoring, protection and control techniques for electric power systems. At present, ring-type current transformers (“CTs”) are the most prevalent technology for measuring phase currents in three-phase electric power transmission and distribution lines. Conventional CTs are often placed in physical contact with the monitored power line conductor, which results in excessive heating of the CT and the power line. Excessive heating can adversely impact electronics in the CT and limit the current carrying capacity of the power line. Conventional CTs are also limited to electric current measurement, resulting in the need for separate voltage sensors when both current and voltage measurement are desired. Conventional CTs also require separate radios to transmit the current measurements to controllers or remote transmission units (RTUs). These radios are typically powered by batteries or separate low voltage wiring. Batteries require periodic maintenance, while low voltage wiring requires a nearby transformer, which increases the cost and maintenance requirements.
Solar storms can cause geomagnetic disturbances that produce direct current (“DC”) currents in electric power transmission systems. These DC currents can saturate grounded transformer windings, which can overheat the transformers and cause voltage instability problems. This leads to power outages and component failure. Conventional CTs are unable to measure the DC component of power line currents, which prevents electric utility system operators from taking appropriate actions. Moreover, conventional protection relays are unable to react to DC current because they are only configured to respond to very high AC fault currents.
High resolution analog-to-digital converters are now available to measure DC currents in the presence of high power alternating current (“AC”) currents. Other techniques can be used to improve the accuracy of DC current measurement. For example, the skin effect of DC versus AC current can be used to separate the AC current signal from the DC current signal. In addition, a 50/60 Hz notch filter with DC gain can be used to increase the accuracy of the DC measurement and flatten the frequency response of the conductor impedance. Another technique uses an AC-coupled signal to remove the DC component and then subtract the AC-only signal from the original. Averaging across the 50/60 Hz power cycle window can also be used to reject the fundamental AC frequency and leave the DC component.
However, these approaches are generally expensive, require complicated signal processing, and prone to cross talk errors from stray voltage sources. As a result, there is a persistent need for improved current sensors for high voltage power lines. There is a particular need for high voltage electric power line monitors capable of measuring DC and AC currents with onboard communication features suitable for placement in many circuit locations in smart-grid applications.